Contains safrole (see also under toxicity) which is a natural pesticide in many plants; 2000 tons a year are used to make commercial pesticides. It is also used in the manufacture of Ecstasy. Used also in manufacture of fragrances, but no longer legally added to food or drink. (see Safrole in Wikipedia)

Ocimum gratissimum contains citral a monterpene alkaloid. There was an increased incidence in malignant lymphomas in female mice, but not in rats or male mice. No definite evidence of genotoxicity found though 'positive results in sister chromatid exchange assay.' Ocimum basilicum contains estragole and safrole, alkylbenzenes, which are carcinogenic and genotoxic in rodents.

van den Berg, S.J.P.L., Restani, et al., P.. (2011). Levels of Genotoxic and Carcinogenic Compounds in Plant Food Supplements and Associated Risk Assessment. Food and Nutrition Science 2: 989-1010. Link